Butter and flour two 9x5x3-inch loaf pans. Beat sugar and oil in large bowl to blend. Mix in eggs and pumpkin. Sift flour, cloves, cinnamon, nutmeg, baking soda, salt and baking power in another large bowl. Stir into pumpkin mixture in two additions. Mix in walnuts, if desired.
Divide batter equally between prepared pans. Bake until tester inserted into center comes out clean, about 1 hour 10 minutes. Transfer to racks and cool 10 minutes. Using a sharp knife, cut around edges of loaves. Turn loaves out onto racks and cool completely. Completely cover loaves in foil to store.